The mechanism of transduction of light into an electrical potential change in vertebrate photoreceptors remains unknown. The project described here proposes to investigate the role of Ca and cGMP in phototransduction. A previously observed light-activated release of Ca by intact rod photoreceptors in the toad retina is to be further characterized. This ion release is studied with the use of specially designed Ca-selective membrane electrodes. The characteristics of this release process in light-adapted and dark-adapted photoreceptors will be compared. The spatial spread in the rod outer segment of the light-activation and light-activation of the Ca release will be investigated, as well as the molecular mechanisms of the release. This Ca release may reflect a change in intracellular free Ca concentration, and this proposition is also to be experimentally tested. Through the development of enzyme-electrodes specific for cGMP, we propose to measure in intact rods changes in the intracellular levels of this nucleotide in response to light. Finally in retinas containing only cones, perhaps Iguana, we will investigate whether Ca ions play a role in phototransduction in the cone comparable to that observed in the rods.